Industrial doors are used in commercial or industrial buildings to separate two zones of different ambient conditions. For example, industrial doors may be used to separate two zones which have different temperature or humidity conditions, or to provide noise control between two zones and have the advantage of being moved rapidly between the open and closed positions.
In a typical roll-up door, as described in U.S. Patent Application Ser. No. 07/371,264, Filed June 26, 1989, a flexible door panel is wound on a drum located above the doorway in the building and is moved between a rolled open position and an unrolled closed position by a hydraulic motor which is operably connected to the drum. As disclosed in the aforementioned patent application, a counter balancing mechanism, such as a counterweight, is attached through a first belt and pulley arrangement to the drum and exerts a force on the drum in a direction to wind the door panel on the drum. In addition, a second belt and pulley arrangement is also secured to the drum and includes a resilient member, such as a spring, which is arranged to exert a force on the door panel to stretch the door panel at all positions.
The conventional roll-up door includes a rigid horizontal beam, formed of metal, which is attached to the lower edge of the flexible door panel. The operation of the roll-up door is normally initiated by the operator of a fork lift truck, either by the operator pulling on a suspended cable, or by the fork lift truck passing over a floor treadle. In case of a power failure, or in a situation where the fork lift operator may not fully actuate or pull the cable, or in the event the fork lift operator does not properly judge the speed of the fork lift with the opening rate of the door, the fork lift truck may engage the door panel and beam with substantial impact. The impact can bend or deform the beam, or rip the fabric door panel away from the beam, or damage the guide mechanism in the side frames of the door. As the conventional beam is formed with a non-uniform cross section, a bent or deformed beam will normally retain its bent configuration and therefore cannot readily be bent back to its original configuration.
Because of this problem, it has been proposed to incorporate a breakaway mechanism with the rigid beam on a roll-up door. A typical breakaway mechanism has included a projection or extension on the side guides which are received within slots in the end of the rigid beam. In one form, each extension has a T-shape and the beam end has a mating "T"-shaped slot. This construction will transmit vertical force but will release if the beam is subjected to a transverse force or impact. In another form, the extensions are maintained within the slots in the beam end by a ball and detent arrangement which will permit transverse release if the beam is subjected to a substantial external impact.
While breakaway devices of this type protect the door and rigid beam against a major or substantial impact, it has been found that they tend to release or break away too easily, meaning that the mechanism may release under high wind pressure or under a minor external bump. This type of release is an annoyance in that it is time consuming and requires a workman to manually re-engage the guides with the beam ends.